Several characteristics are important in mobile electronic devices. Among these are durability, simplicity, resistance to environmental conditions, low cost and low weight. Commonly, such electronic devices incorporate display devices, such as printers, for providing a visual image of data present in the devices. One area where this is useful is in devices used for electronic monitoring of patients in medical situations.
A strip chart recorder is often employed to provide a printed record, particularly for medical electronic devices. As data is output, a continuous stream of paper output is produced. This can occur, for example, when continuously monitoring a patient's physiological responses.
In mobile devices intended for use with exposure to all kinds of environmental conditions, inclusion of a paper output can provide easy access for moisture and other contaminants which can impair device performance. In particular, if the paper strip becomes moist at, or near, the printing location, the printing process can be significantly impaired. Moreover, when the paper becomes moist, drag between the paper strip and any surfaces with which it comes in contact may be increased significantly. This can disrupt the flow of paper and cause bunching or jamming of the output paper stream.
One possible technique for reducing this problem is to output the paper strip from the bottom of the mobile device. This can lower the likelihood that moisture in the form of rain or condensation will enter the device from above. However, this technique is impractical where the mobile device may be used on surfaces with debris or water build-up. Further, an output paper stream from the bottom of the device is often less accessible to the operator than a paper stream from the top or side of the device.
Regardless of which location is chosen for the output, the paper strip must be delivered from its supply location to an accessible output location where the data on the paper can be viewed and a length of paper can be removed from the device. Preferably, paper can be supplied from within the device from a source located near to the output to minimize the problems of transporting the paper. Because size, weight and other design constraints of mobile devices may limit the feasibility of locating the paper source near the output, this may not always be achievable. As a result, the paper strip often must be directed through some distance before exiting the device. Even where proximate location is feasible, some redirection of the paper strip may be desirable to allow the paper strip to be output at a desired location and angle.
Due to the lack of substantial rigidity in typical paper strips, transportation of the paper strip from the paper source to an output location presents problems. An extended length of paper is not easily pushed across any substantial distance, either upwardly or laterally, without significant bending. This presents a difficulty analogous to trying to push on a string. Rather than traveling toward the output, the paper doubles over itself. Increasing the pressure on the paper strip provides no benefit.
The paper strip's lack of substantial rigidity becomes even more problematic when the paper is to be directed through a defined angle of curvature from an upward direction toward the output location. As the paper is bent through the angle, the upward force on the paper strip is not transmitted well through the curve and does not translate into a lateral longitudinal force. Consequently, increasing upward longitudinal force serves only to increase the bending problem in the paper causing deviation from the desired angle of curvature.
One approach to these problems of delivering the paper strip from within the device to an output location, either laterally or vertically located with respect to the paper source is a mechanical aid such as mechanical rollers, either free or actively driven by the device. In mobile devices, this may result in added complexity and weight. It may also increase the difficulty of replacing a paper source in the field.
Alternately, the paper source may be designed such that the paper exits the paper source in substantially the same direction as the paper output. A lower support can be used supplementally to limit paper sagging. Where paper drag along the lower support is significant, this approach may still result in paper bunching. Moreover, because the paper stream usually must pass a printing element, such as a thermal printhead, the paper source orientation may be dictated by considerations relating to the printing source. For example, it may be desirable to pass the paper stream past the printhead vertically to minimize debris build-up between the printhead and the paper stream or to reduce effects of vertical forces, such as gravity or vibration.
In such cases, it is often desirable to use a mechanical guiding system which redirects the paper stream from the output of the printing source through a radius of curvature toward the desired output location. Addition of a mechanical guide often adds weight and complexity to the device. Further, as the paper stream is directed through a radius of curvature, it rubs along one surface of the guide. Friction between the paper stream and a surface of the guide may cause bunching of the paper stream, resulting in a paper jam within the guiding mechanism.